But the
scheme which is large enough to embrace the activities of the highest
form of life, covers all those of the lower creatures. The lowest plant,
or animalcule, feeds, grows, and reproduces its kind. In addition, all
animals manifest those transitory changes of form which we class under
irritability and contractility; and, it is more than probable, that when
the vegetable world is thoroughly explored, we shall find all plants in
possession of the same powers, at one time or other of their existence.
I am not now alluding to such phaenomena, at once rare and conspicuous,
as those exhibited by the leaflets of the sensitive plants, or the
stamens of the barberry, but to much more widely spread, and at the same
time, more subtle and hidden, manifestations of vegetable contractility.
You are doubtless aware that the common nettle owes its stinging
property to the innumerable stiff and needle-like, though exquisitely
delicate, hairs which cover its surface. Each stinging-needle tapers
from a broad base to a slender summit, which, though rounded at the end,
is of such microscopic fineness that it readily penetrates, and breaks
off in, the skin. The whole hair consists of a very delicate outer case
of wood, closely applied to the inner surface of which is a layer of
semi-fluid matter, full of innumerable granules of extreme minuteness.
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